1. Field of the Invention
The present invention relates in general to measuring systems for high frequency communication systems and specifically to measuring devices for measuring passive intermodulation.
In particular, the present invention relates to a method for measuring intermodulation generated in a signal transmission path in which a first HF signal u1(t) with a predetermined frequency progression and a second HF signal u2(t) with a predetermined frequency progression are generated and the first HF signal u1(t) and the second HF signal u2(t) are introduced into the signal transmission path, wherein an intermodulation signal containing intermodulation products is produced in the signal transmission path from the first HF signal u1(t) and the second HF signal u2(t). Such an intermodulation signal can for example contain indications of the location of defective points in the signal transmission path and can therefore be used to locate faults. The invention further relates to a measuring device for carrying out such a method.
2. Description of Related Art
Description of Passive Intermodulation (PIM)
The quality of a connection between a permanently installed transmitting and receiving installation (BTS, base transceiver station) and a terminal device (UE, user equipment) plays a key role in modem mobile networks. Due to the high power generated in the BTS on the one hand and the necessary sensitivity of the receivers of the BTS and. UE on the other hand, faults in the transmission path can significantly influence the sensitivity of the receivers and thus the quality of the connection.
One key effect which gives rise to faults in the transmission path is intermodulation. For example, through intermodulation, two transmission signals with two different carrier frequencies which are generated with high power in a BTS generate interfering signals at points with non-linear transmission behavior (often simply referred to as “non-linearities”), the frequencies of which are sums and differences of integer multiples of the frequencies of the transmission signals.
Some of these interfering signals can fall within the reception band of the BTS and thus adversely affect the quality of the communication. If these interfering signals are generated in passive elements, this is referred to as passive intermodulation (PIM).
FIG. 1 is a schematic representation showing a signal transmission path from a BTS up to an antenna. The BTS 10 is connected with the antenna 13 via a first filter 11 and a second filter 12. The BTS 10, the filters 11 and 12 and the antenna 13 are connected together via high-frequency cables 14, 15 and 16, which are connected to the respective elements via high-frequency connectors 17 to 22.
PIM can occur in all components 11 to 22 of the transmission path. For example, corrosion in plug connectors, oxide coatings on contacts and metal-metal transitions, impurities in materials and insufficiently fastened plug connections can cause PIM.
In order to ensure and check the quality of the transmission device, and/or in order to locate such faults, measurements of the PIM are carried out. Since PIM occurs in particular at high powers, as a rule this is measured with the use of high transmitting power, for example 2*20 W.
Structure of Conventional PIM Measuring Devices
Special measuring devices are available for the measurement of PIM. A conventional PIM analyzer is represented by way of example in FIG. 2. It consists of a control unit 151 and a signal unit 161. In the signal unit 161, high frequency signals with suitable different frequencies f1 and f2 are generated in the signal sources 113 and amplified in the two power amplifiers 114. In the combiner/adder 115, the two transmission signals are combined and sent to the device 130 which is to be tested (device under test, DUT). The PIM occurring in the DUT 130 is selected in the filter 116 and detected and measured in the measuring receiver 117. The control, analysis of the measuring results, and representation take place in the control unit 151.
Such a device is for example described in the publication DE 10 2010 015 102 A1.
Problem Which is to be Solved Through the Invention
However, it has been found that, due to noise and other interference signals or undesired signal components in the intermodulation signal, the method and measuring devices described above often do not permit satisfactory quality testing of signal transmission paths. In particular, the spatial resolution achieved in the location of faults is often inadequate.